New shed floor, need suggestions...

Old 07-30-18, 08:43 AM
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New shed floor, need suggestions...

I've installed a new 10x14 shed floor which is constructed of pressure treated 2x4s 16" on center, perched atop 12 concrete blocks. We had a lot of rain between the time I finished the floor frame and the time I put the 1/2" plywood down, so things are pretty damp still.

Now that the plywood is down, it feels spongy. I'm not sure if it's due to the joist spacing or just because the plywood is still pretty wet.

My question is: should I add another layer of 1/2" plywood? If so, does it have to be PT or should I put a vapor barrier on top of the PT base layer, and then the non-PT layer on top? The top layer of plywood will be enclosed withing the shed. If I add another layer, I will stagger joints and not screw into the joists; but should I use an adhesive between?
Old 07-30-18, 08:50 AM
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1/2" is bit thin for a floor but might be ok for a shed [depending on it's use] If I understand you correctly you used 2x4s for floor joists ?? that is too small! 2x6s would have been on the small side. Is it feasible to either sister 2x6s along the 2x4s or add more support under the 2x4s?
Old 07-30-18, 09:12 AM
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Is the sponginess from the plywood or from the structure underneath moving? As Marksr mentioned both your floor framing and sheeting are under sized. If it's just the sheeting I would add a second layer of 1/2". I would not install a vapor barrier so the wood can dry more easily.

If it's the floor joists bouncing they can be stiffened if you can get some blocks in the middle to break up the span. And, most 2"x lumber is NOT treated for ground contact you will want to maintain good airflow underneath the shed to help the wood last as long as possible.
Old 07-30-18, 07:41 PM
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Thanks for the replies. Yes the frame is raised up off the ground on the "pyramid" style concrete blocks. The spongy feeling is between the joists, other than that it feels solid. 2x4s were recommended along with the spacing of 16" on center in the shed's manual so that's why I went with that.

No, I cannot sister additional 2-bys to the frame (see layout below).I'm surprised that "2x6s would have been on the small side", I would have thought that was overkill.

The shed will be light duty AKA tool storage, outdoor toys, etc. I won't be parking my riding mower inside.

Here's the layout of the frame/floor (120" x 160"):

Old 07-31-18, 03:03 AM
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The way you have the support blocks I would suggest that you use 3/4" plywood.
Your blocks are only supporting every second joist which is why 2x6 or wider might have been a better choice.
A different way of laying out the blocks is to use 2 or 3 - 4x4's from front to back then laying the joists across them.
This way every joist would be supported and the extra height would give you more airflow underneath.
Old 07-31-18, 04:39 AM
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You should really consider rebuilding, the 5' span needs much stiffer structure, I'd even consider 2x8 and 3/4 ply.

You already can see what the results are fix it now while still an option.

You may not plan to park anything heavy now but who knows what will change in the future.

You will be able to use the material for the walls so not a total loss!

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